This invention provides composite dental restorative compositions which are curable by the action of light having visible and/or ultraviolet components. More particularly, formulations are disclosed which maintain good color stability and other beneficial physical properties while having the ease of workability attendant to light curable materials. These compositions employ broad spectrum photosensitizing systems having surprising synergistic superiority over either ultraviolet curing or visible light curing systems alone.
Dental restoratives should exhibit certain obligatory physical and chemical characteristics in order to be suitable for use in filling, repairing or replacing teeth. These restorative materials should possess properties that closely match natural teeth with respect to structural properties such as cohesive strength, coefficient of thermal expansion and wearability. Also, aesthetic considerations such as color stability, refractive index, plaque repellency, polishability and opacity are important factors in determining whether a material is suitable for use as a dental restorative. In the past, numerous organic composition have been tried in various mixtures and proportions in order to find satisfactory materials for use as dental composites or restoratives. These compositions have usually included some type of resin, which may either be preblended or mixed by the practitioner in the office, together with other materials such as pigments, catalysts, handling agents and opacifiers. For restorative use, it is generally necessary to employ materials which are "filled", that is, to which have been added amounts of inorganic, or in some cases, organic particulate material.
Composite or restorative materials should be distinguished from most film forming dental compositions. Such film forming materials provide resins containing minor or no substantial amounts of filler materials. They are frequently used as sealants, glazes, bonding agents, or adhesives. They may be used to coat a prepared tooth cavity prior to filling, thereby sealing off the tooth material against cracks and leaks adjacent to the filling. Such unfilled compositions have different viscosity requirements from dental composite materials because low viscosities are needed in the sealant materials in order to have proper flow characteristics. By contrast, dental composite and restorative materials must have good forming characteristics so that they can be shaped to fit a cavity area or molded into place in order to repair chipped or damaged teeth. Furthermore, such restorative compositions must preferably be filled with inorganic materials in order to achieve satisfactory hardness and durability during service.
It will be appreciated by those skilled in the art that the use of photoactivated materials is to be preferred over the more traditional thermochemical catalyst or redox activated systems because of the increased work time allowed by the use of photoinitiated polymerization. In a two component catalyst or redox system, work time is determined by the reaction time once the catalyst is added to the resin component. In a photocured system, the practitioner may take whatever time is necessary for forming or molding the dental restoration into formation and then effect extremely rapid curing by exposing the photocurable material to the appropriate wavelength of electromagnetic radiation.
Some prior art dental materials have utilized photoinitiators that are sensitive to ultraviolet radiation. There are, however, certain technical limitations which are present in ultraviolet-activated dental composite or restorative systems. For example, tooth structure attenuates ultraviolet radiation sufficiently so that it is not practical to cure ultraviolet-activated dental composites where direct access to the dental composite by the ultraviolet source is interfered with by intervening portions of tooth structure. Such is the case is classical undercuts used for mechanical retention of dental restorations. Ultraviolet-cured systems also cannot accomplish good depths of cure; stepwise restoration is frequently required.
It has been found that visible light having wavelengths from about 4000 angstroms to about 5000 angstroms is attenuated to a lesser degree by tooth structure than is ultraviolet radiation. Accordingly, it has been proposed to employ such visible light as the source of activating energy in dental compositions. Many previous attempts to develop restorative formulations using visible light curing systems have resulted in failure; such previous compositions have failed to exhibit one or more of the serviceability characteristics necessary for dental restoratives. A principal shortcoming is a lack of color stability and concomitant lack of aesthetic acceptability of the resulting products. Such a lack of color stability is a major shortcoming. The American Dental Association, the Internatinal Standards Organization, and others have developed detailed requirements for color stability in direct filling resins. See, for example, A.D.A. Specification No. 27; J.A.D.A. vol. 94, June 1977, pp. 1191-1194.
Accordingly, it is a principal object of this invention to provide compositions which are useful as dental restoratives. It is another object to provide such dental materials which exhibit improved color stability. Another object of this invention is to provide a one-component photocurable dental restorative system which is photocurable using light having ultraviolet and/or visible components. A further object is to provide dental compositions which exhibit a rapid cure time but which exhibit good workability prior to curing. Yet another object is to provide such compositions which exhibit improved shelf stability. A still further object is to provide dental compositions which are effective with lesser amounts of photoinitiators. Still other objects will become apparent from the following description of the invention.